The invention outlined in this application relates to discharge lamps, specifically to those in which dielectrically impeded discharges burn during operation. In such discharge lamps, which are frequently denoted as silent discharge lamps, discharges are generated in a discharge medium with the aid of a set of electrodes. Dielectric impediment is produced by a dielectric layer between at least a part of the electrode set and the discharge medium, this part consisting at least of the anodes when the distribution of the tasks of the electrodes is fixed.
The details relating to silent discharge lamps need not be set forth here, because they belong to the prior art. Silent discharge lamps have recently been given increasing attention because it is possible with the aid of a special pulsed mode of operation (U.S. Pat. No. 5,604,410) to achieve relatively high UV efficiencies that permit economic generation of visible light given the use of appropriate fluorescent materials. The invention relates both to UV radiators and to lamps with visible emission. Of particular interest in this case are flat discharge lamps which can be used, for example, for backlighting displays, monitors and similar devices. Such flat discharge lamps generally have a plate-like design, that is to say they have a base plate and a top plate which define a discharge space between them for the discharge medium. At least one of the plates must be designed for light emission, the top plate being considered here as at least partially transparent. Of course, the top plate can in this case bear a fluorescent material which is not itself transparent in the true sense.
Because of the flat design, problems with mechanical stability arise in the case of relatively large formats of the flat discharge lamps. Consequently, it has become established to use support elements between the base plate and top plate. These support elements connect the two plates and thereby shorten the bending length between the outer edges of the plates on the paths between the support elements. In the outer region, the plates are generally connected via a frame enclosing the discharge space, which is not denoted as a support element here, although it also connects the plates and has a supporting function. The number of support elements is determined by the requirements placed on the loadability in bending and in compression, as well as by the format of the lamp, of course.
The invention is based on the technical problem of specifying a silent discharge lamp of the type described at the beginning having an improved mechanical design.
The invention provides for this purpose: a discharge lamp having a base plate, a top plate for the light exit, which is at least partially transparent, a discharge space between the base plate and the top plate, for holding a discharge medium, an electrode set for producing dielectrically impeded, individual localized discharges in the discharge medium, a dielectric layer between at least one part of the electrode set and the discharge medium, and a multiplicity of support elements which produce a connection between the base plate and the top plate, characterized in that, apart from those at the edges of the discharge space, the individual discharge regions are surrounded by in each case substantially identical patterns of support elements.
The invention also relates to a display device with such a discharge lamp, for example to a flat display screen, a display or a similar device using LCD technology.
The essential idea of the invention resides in not, as in the prior art, using the support elements in as small a number as at all possible but, on the contrary, distributing a relatively large number of support elements over the surface of the flat discharge lamp. The inventors have verified that, given appropriately more frequent support, it is possible to use comparatively thin base plates and top plates such that it is possible to realize a substantial weight saving for the overall lamp. The overall weight of the lamp is, however, of substantial importance for many applications. Moreover, in the case of relatively light plates the mounting method and automatic mounting devices possibly required therefor can be rendered substantially more simple and less expensive. Lighter plates are, moreover, associated with lower thermal capacitances, so that thermal cycles can be traversed more quickly, thus further simplifying the production. Moreover, it is of course also possible to achieve improved stability with a larger number of support elements.
In this case, the support elements, which can themselves certainly be multipartite, but are preferably unipartite, are to be arranged in an assignment relating to individual localized discharges in the discharge space. It is firstly to be stated in this regard that the individual localized discharge structures have appeared with the already mentioned pulsed operating method even without this invention and were able to be permanently located by creating preferred sites on the electrodes. However, the invention is not restricted to lamps with such preferred sites. Rather, it transpires that the invention itself results in preferred locations between the support elements for individual discharges, so that for example conventional structures, for example nose-like projections on the cathodes, can also be less strongly pronounced. To the extent that individual discharge structures can be produced between the support elements according to the invention independently of the possible pulsed operating method, the invention also relates thereto.
To the extent that this application talks of individual discharges or discharge structures, these statements relate, strictly speaking, to regions prescribed by the design of the lamp, in particular of the electrodes and the supporting projections, in which such individual discharge structures can burn. Depending on the operating state of the lamp, however, variously extended discharge structures are also conceivable in this case within these regions. Thus, the regions need not necessarily be filled entirely with a discharge structure. Above all, the desire can be to influence the size of the discharge structures in conjunction with dimming functions of the lamp. The statements in this application therefore relate to the regions which can be filled to the greatest extent with discharge structures. To the extent that electrode structures are provided for fixing preferred positions of discharges, there will generally be a 1:1 correspondence with the discharge regions.
The assignment between supporting projections and individual discharge regions is to be present in the invention at least in so far as the individual discharge regions are respectively surrounded by identical patterns of directly adjacent supporting projections. This excludes, of course, discharge regions in the edge region of the discharge lamp, that is to say in the vicinity of the frame or the lateral closure of the discharge vessel. The aim in this case is to design the pattern of the directly adjacent supporting projections around a discharge region together with this discharge region so as already to homogenize the luminance here as far as possible. The relatively large number of supporting projections then does not play a disadvantageous role for the homogeneity (compare the above explanations on the overall design of the discharge lamp). Of course, individual supporting projections can be directly adjacent to more than one discharge region, and this will even be the rule. It is also preferred that the supporting projections for their part are surrounded as far as possible by the same pattern of directly adjacent discharge regions in each case.
Moreover, the assignment between support elements and individual discharge regions is intended in the invention preferably to be present to such an extent that it is possible to find a plane through the discharge space between the base plate and top plate and a direction in this plane along which the support elements and the discharge regions alternate. The alternating row need not be a row alternating directly one after the other (according to the pattern ababab . . . ). Also included is a row in which two support elements or two discharge regions occur regularly one after another as long as each support element and each discharge region has at least one discharge region or at least one support element as its neighbor (that is to say, for example, abbabbabb . . . or aabbaabb . . . )
They need not necessarily be strictly collinear in this direction of the alternating row, but can also be distributed in a somewhat zigzag fashion. It is preferred for a multiplicity of such rows which are parallel to one another to exist in this plane. It is also preferred for there to be in the plane a second direction which is not situated parallel to the first-named direction and along which there is likewise an alternating row of support elements and discharge regions. In this case, there is preferably both a set of parallel rows in the first direction and a further set of parallel rows in the second direction. Consequently, the overall result is a planar pattern of support elements and discharge regions of alternating design, for example a chessboard pattern.
Moreover, it is preferred in the above definition that the straight line along which the alternating row results connects the centers of directly adjacent discharge regions or discharge regions which are at most situated next but one or the centers of directly adjacent support elements or support elements situated next but one.
A further idea of the invention consists in no longer, as in the prior art, understanding the support elements as optical disturbances in an overall discharge structure that is otherwise designed as homogeneously as possible. Rather, according to the invention the aim is to regard the support elements in their now relatively large number as an integral component of the structure responsible for the final luminance distribution. Consequently, the overall structure of the individual discharge regions is optimized together with the support elements and the optical modifications effected by them. In this case, as long as they are surrounded by a sufficiently large number of discharge regions, regularly occurring shadings can in principle be compensated just as effectively by diffusers or other homogenizing measures as was the case conventionally for the few support elements used. Moreover, as explained in more detail further below, the support elements can, however, themselves also be used for homogenization, for which purpose they preferably consist of optically transparent material. The supporting projections can certainly also be provided with a fluorescent coating, but they can also (by contrast with the remainder of the top plate) be entirely or partially free from fluorescent material, for example be wiped free subsequently. They can additionally be brightened up thereby, because the unavoidable extinction of the fluorescent layer is eliminated. For the above reasons, the invention provides that the support elements and the individual discharges, apart from edge effects of the lamp, in each case have substantially identical surroundings, that is to say, for example, all the support points are surrounded by an identical pattern of directly adjacent discharge regions, or vice versa.
In the case of electrode sets with strip-shaped electrodes which, apart from local structures (preferred points for discharge regions), run more or less rectilinearly, it is preferred that the discharge regions on a respective side of a specific electrode strip are separated in each case by support elements, for example alternate in each case with support elements, that is to say support elements are provided in each case between the discharges. A particularly simple example is chessboard-like overall arrangements of support elements and discharge structures. The exemplary embodiments illustrate this, but also show a counter example.
Overall, consideration is preferably given to intermediate distances between directly adjacent support elements which are 30 mm or less. In the case of typical dimensions of discharge paths and transverse extents of individual discharge structures, optically favorable and very stable support element patterns can be formed in this region.
According to a further point of view of the invention, the support elements are designed as supporting projections in the sense of a unipartite component of the top plate, the outer contour tapering toward the base plate in at least one cutting plane perpendicular to the base plate. The invention is thereby delimited from conventional support elements which, in the relevant prior art normally had the form of glass balls separating the plates. The supporting projections, according to the invention, of the top plate can already be provided during the production of the top plate as a moulded element of the top plate, for example by thermoforming, pressing or another suitable shaping method. In principle, they can also be integrally moulded subsequently, although in this case they are to be designed in one piece with the top plate when the lamp is actually mounted, so that the previous substantial outlay for the positioning and fixing of separate support elements between the plates can be eliminated. The outlay on mounting would otherwise be substantial precisely with the large number of supporting projections according to the invention. However, by way of example, it can also be sensible for the purpose of fastening the supporting projections on the base plate to provide a connecting elementxe2x80x94for example made from solder glassxe2x80x94between the base plate and the supporting projections.
An integral production with the top plate is, of course, most favorable in this case. An advantage of this unipartite design with the top plate by contrast with being an integral part of the base plate resides in that the contact between a supporting projection and a plate unavoidably produces certain shadows in the luminance distribution which can impair the homogeneity and must be compensated. According to the inventors"" findings, this compensation is easier the further removed the contacts causing the shadows are removed from the light emitting side of the top plate. This holds, in particular, in the case of the use of diffusers and other homogenizing elements on the top side or above the top plate. The greater the distance from such homogenizing elements, the better the possibilities of optical resolution of the shadows. The already mentioned tapering contour of the supporting projections should occur in at least one cross-sectional plane, the cross sectional plane running perpendicular to the base plate. The perpendicular orientation is to be defined locally in the case of a non-planar base plate. Because of the taper, the supporting projection is narrower in the direction along the plates just above the base plate than it is further removed from the base plate. This taper preferably effects the entire height of the supporting projection. However, not all the existing supporting projections need necessarily be provided with the shape explained here.
These supporting projections that are slimmer in the region of the base plate at first exhibit relatively small shadow effects. In the case when the individual localized discharge structures are produced above the base plate, it is thereby possible also to keep a space free for the discharge structures by virtue of the fact that the latter can exist largely without being influenced by the supporting projections. The discharge structures can then be moved together with a way that is favorable for the homogeneity and be arranged with a high density with the aid of which high luminances can be generated. Finally, the tapering contour can also generate favorable optical properties of the top plate, something which will be described further in more detail. The favorable optical properties lead in the way already outlined at the beginning to the fact that the larger number of supporting projections contributes to the homogenizing as an integral component of the lamp design, and need not be understood as disturbance of a structure homogenized independently of the supporting projections.
In order to avoid additional shadings and to utilize possible positive optical effects of the supporting projections, the latter preferably consist of an optically transparent material. However, they can in this case be coated entirely or partially with a fluorescent material, as is also the case with the remaining top plate. The supporting projections and the remainder of the top plate preferably consist of glass.
The shaping of the supporting projections is preferably designed such that not only is a cross-sectional plane with a tapering cross section produced, but, moreover, there is also no cross-sectional plane in which the supporting projection widens too substantially in the direction of the base plate. When expressed in other words, this means that the outer surface of the supporting projections faces the discharge space of the base plate, in any case the important part of the outer surface. There can also be individual regions of the outer surface which run perpendicular to the base plate, but not over an important part of the circumference of the supporting projections. In this case, the outer surface extends from the base plate up to the top plate, and so there is no talk here of a small part region of the outer surface.
The outer surface of the supporting projection is intended to form, in relation to a plane that cuts the supporting projection and runs at least locally parallel to the base plate between the top plate and the base plate, an angle of preferably at least 120xc2x0, better at least 130xc2x0 and, in the most favorable case, 140xc2x0 or more, this angle being defined in a cutting plane perpendicular to said plane and in the direction of the base plate. The angle thus refers, as an obtuse angle, to an outer surface of the supporting projection tipped toward the base plate. With such oblique outer surfaces, space for the discharges can still be created in the vicinity of the underside of the supporting projection adjacent to the base plate, on the one hand, but on the other hand these oblique outer surfaces are important for possible optical functions of the supporting projections.
Specifically, when the supporting projections according to the invention are limited by the obliquely running outer surfaces described, through refraction of light impinging from the discharge space, or through appropriate alignment of the emission characteristics, of a fluorescent layer from the outer surface, they ensure an alignment of light into the core region of the supporting projections. It is thereby possible to counteract the shadows produced by the contact with the base plate.
Furthermore, together with a pattern, prescribed by the electrode structure, of individual discharges it is possible to undertake an optimization to a luminance that is as homogenous as possible in an overall design of the arrangement of supporting projections and of the discharge structure. In addition to the shading effect of the contact between the supporting projection and base plate, it has also specifically to be taken into account that the individual discharge structures typically burn not below, but between supporting projections. Consequently, the maxima of the UV generation are likewise situated between the supporting projections. As a result of the effect of optical deflection, the light can be brought partly from these regions into the regions of the supporting projections so as to produce a relatively homogenous luminance on the top side of the top plate. The aspect of the invention addressed here is brought out more vividly by the exemplary embodiments.
As already touched upon, the supporting projections are to taper in the direction of the base plate. It is optimal in this case when the supporting projections are as narrow as possible in the region of the contact with the base plate, the term xe2x80x9cnarrowxe2x80x9d being measured in relation to the other dimensions of the supporting projection. xe2x80x9cNarrowxe2x80x9d is in this case a path forming a small fraction, for example less than ⅓, xc2xc or ⅕ of a typical transverse dimension (along the plates) of the supporting projection, for example half the height of the discharge space. This narrowness should be present in this case in at least one direction, but preferably in two directions in the xe2x80x9clocalxe2x80x9d plane of the base plate. In other words, it can be a linearly narrow or approximately punctiform contact surface.
Very generally, even in the case of somewhat larger bearing surfaces in relation to the base plate, the supporting projections can run substantially like ribs along the top plate, or be limited to small regions in relation to the dimensions of the plates. In the first-named case, it is the linear contact surfaces that are the general concern for narrow contact surfaces, while in the second case it is the approximately punctiform ones. The rib-like supporting projections can have specific stabilization functions, for example they can provide the top plate with an improved motability in bending in one direction. Furthermore, as will be explained in still further detail in the exemplary embodiments, they can also serve to separate specific regions in the discharge space from one another, in order to influence the discharge distribution. Thus, together with the electrode structure they can define preferred locations for individual discharges and separate individual discharges from one another along identical electrodes. On the other hand, the supporting projections limited locally in two directions in the plane of the plate offer the possibility of minimized shading effects, and are generally sufficient for the support function.
A preferred shape for locally limited supporting projections can therefore be formed by a cone or by a pyramid, in the case of which the vertex touches the base plate (and is in this case possibly somewhat flattened off or rounded). In principle, any desired basic shapes come into consideration for the cones and pyramids, that is to say surfaces limited with any desired curves, polygonal surfaces or mixtures thereof. However, it is largely supporting projections without edges, that is to say cones, that are preferred, because the edges can lead to certain irregularities in the light distribution. As already stated, an attempt is to be made to keep the contact surfaces between supporting projections and base plate as small as possible. Limits can exist in this case that are set by production methods (rounding in the case of glass shaping) or by the mechanical point loading of the base plate, so that rather than a supporting projection actually coming to bear xe2x80x9cin a pointed fashionxe2x80x9d against the base plate, there is a slight rounding or flattening off. As long as this rounding or flattening off is not of any substantial consequence in relation to the size dimensions of the supporting projection, the basic idea of the narrowness is not thereby impaired.
However, the preferred feature of the invention is to keep the contact surface between the supporting projection and the base plate as small as possible by virtue of the fact that it results only from bearing by touching. In other words, instances of bonding, solder glass and the like, which would necessarily enlarge the contact surface somewhat, are to be dispensed with as far as possible. For the rest, such additions usually have the disadvantage that they release gases upon heating during lamp production so that extensive pumping operations are required to keep the discharge medium pure. Production is substantially simplified if, in accordance with the invention, such substances are dispensed with. However, it is not excluded in the case of bearing by touching that the supporting projections can be pressed slightly into other layers that are required in any case, for example into reflection layers or fluorescent layers on the base plate. A similar statement can hold for a fluorescent coating of the supporting projections themselves.
This bearing purely by touching between supporting projections and base plate generally suffices for the targeted stabilization effect, because mechanical stresses pressing the plates away from one another do not occur, as a rule. This holds, in particular, for the case, which is of most interest technically in any case, in which the discharge lamp is operated with a discharge medium at low pressure. The supporting projections are then pressed against the base plate by the external overpressure.
Finally, in the case of this invention preference is given to such discharge lamps as are designed for bipolar operation, in the case of which the electrodes therefore function alternately as anodes and as cathodes. Owing to a bipolar operation, the discharge structures, which are inherently generally asymmetric, are superimposed on one another to form a symmetrical distribution on average over time, for which reason the optical homogenization can be further improved.